Info on BCDD - Boost Controlled Deceleration Device

[MattSaint]

Howdy folks,

I was able to finally track down a BCDD (Boost Controlled Deceleration Device) for my early (9/75) 1976 280Z and wanted to share with the community as well as ask some questions.

Here is a picture of the device for an early 76:  Note that the altitude corrector is flush to the bottom of the outer housing and is similar in appearance to a set screw.  On the side you can see the screw which locks it in place.

Note the orientation of the screws, 3 are used to attach the BCDD to the Throttle Body, 3 are used to keep the sections of the BCDD where the diaphragms are together and the last one is just to keep everything aligned. I accidentally took mine apart, but thankfully didn't damage the diaphragms.

Here is one of the diaphragms that usually tears.  Before cleaning.

I am currently cleaning everything and will post additional pictures.

Edited October 17, 2017 by MattSaint
terminology

[MattSaint]

One of the questions I have for everyone, is should I assemble this dry?  There was some oil, grit and grime in the mechanism?  and If yes, what should I use?

Thank you!
~Matt

Edited October 17, 2017 by MattSaint
spelling

[MattSaint]

Ok, so now I really tore into it...

Here is the external housing...

Here is the "altitude corrector" (Capt Obvious... a hermetically sealed air chamber that acts as a little barometer and compensates for changes in atmospheric pressure.)

On the coil end of the altitude corrector is a dowel, which had fallen out, thankfully I found it and returned it to its proper location.

On the receiving end is another diaphragm in which the altitude corrector dowel fits into...

If you flip it over, there is the valve that is adjusted...

More to come...

 

 

Edited October 17, 2017 by MattSaint
Removed uneeded images, spelling and grammer

[Zed Head]

Not sure what you mean by "dry" but I would only use a small amount of sealant around the edges of the diaphragm, if any at all, if it looks like the rubber won't seal.  The device is only meant for air to pass through and be diverted to desired places.

Also hope that you marked your starting points so that you can be close to correct adjustment when you put it back together.  There is at least one thread on the site about adjusting the BCDD, even though Nissan says to never take it apart, just replace it.

@siteunseen

@Captain Obvious

[MattSaint]

Thanks for the reply.

There was oil and such throughout the entire unit and I wasn't sure if I should add oil to all the various pieces/parts or wipe everything down?

I did make a note of the starting positions and can also see where the lock screw was tightened against the adjuster.

I was surprised at the condition of the rubber seals.  The only rubber gasket that I need to replace is the one that is used to mate to the throttle body itself.  Do you think I can make it out of regular gasket material or do I need to find a rubber replacement?

[Zed Head]

The oil would have come from the PCV system.  Nissan redesigned the PCV hose routing in 1977 to avoid blowby fouling of the throttle body.

I'm pretty sure the CO has been in to the BCDD machinations, and that siteunseen has "tuned" his for better operation.  They might have some suggestions.

[siteunseen]

captain's is more suited for disassembly.  I just read the FSM and turned the nut clockwise until the rpms dropped like they should at red lights and stop signs.

 

[MattSaint]

12 hours ago, Zed Head said:

The oil would have come from the PCV system.  Nissan redesigned the PCV hose routing in 1977 to avoid blowby fouling of the throttle body.

I'm pretty sure the CO has been in to the BCDD machinations, and that siteunseen has "tuned" his for better operation.  They might have some suggestions.

Ok, that makes sense.  That is why there was a lot of coking where the altitude corrector passes through the external housing.  Hence the reason the one in my car has frozen and cannot be adjusted.

Edited October 17, 2017 by MattSaint
terminology

[MattSaint]

Thanks siteunseen for the links.  I had read those posts previously when I initially tried to adjust mine.  As stated above, the one in the car is frozen in place.

Any thoughts on the BCDD to Throttle Body gasket?  It appears to be made of rubber similar to the diaphragm, although it has hardened with time.  I tried to carefully cut/scrap it off with a razor blade, but didn't have much success.  Can I just use a good gasket material?  Not sure if it was intended to have some flex to it?

Edited October 17, 2017 by MattSaint
grammer

[Captain Obvious]

Matt,

The two thin flexible rubber "gaskets" are actually called "diaphragms" and they are a little specialized. You can't just replace them with any simple rubber, so I would treat the originals gingerly. Just wipe as much of the oil off and reuse them. The third gasket (where the BCDD mates to the throttle body) I think is just regular gasket material and can be replaced with a home made version, but the other two cannot.

The "adjuster coil" is actually a hermetically sealed air chamber that acts as a little barometer and compensates for changes in atmospheric pressure. Datsun called it the "altitude corrector", and it's meant to account for changes in pressure due to altitude.

As for assembly, everything should be put together dry. In fact, there shouldn't be any oil anywhere in the BCDD, but as mentioned earlier, the oil gets there accidently (and probably unavoidably) through the PCV system. I don't think you can prevent all of it. Clean everything off, dry it out, put it back together, and hope for the best?

[MattSaint]

Capt Obvious,

Thank you very much for the reply, insight and corrections.  I will correct my language above so the terminology is correct for future reference.  It is much appreciated.

I started this thread for a several reasons.  The first was that I wanted to give back to the community for all the help I have been given.  The second was there is still, after all these years, an air of mystery around the internals of the BCDD.  I took this one apart by accident, but was glad that I did because it needed a good cleaning.  I am just very thankful that I didn't ruin the diaphragms in the process.

I will take what is left of the BCDD to Throttle Body gasket to the local parts store and get as close a replacement as I can.  Once I get it installed I will let everyone know the results.  Fingers crossed.

[MattSaint]

An interesting find...

After cleaning and reassembling the new (to me) BCDD, I decided to take mine off of the car.  I was able to separate it easily from the Throttle Body, the difficult part was separating it into it pieces.  The Altitude Corrector was frozen to the outer housing so badly that I had to use a screw driver and hammer to get it out.  When I finally did I was surprised to discover that there was a spring!  The other one didn't look like it had ever been taken apart, but there was no spring inside that is for sure.  The only differences to note was that mine had a number 8 stamped on the housing and the other had a 7.  Thoughts?